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Powerwall Backup Sizing Calculator

Choose the circuits you want running when the power goes out, set how long you need them to last, and see how many Tesla Powerwall units it actually takes. Built by a Tesla Energy Certified Installer and licensed C10 electrician in Los Angeles.

Tesla Energy Certified Datasheet-Accurate Specs Energy, Power & Surge Modeled Built by a Licensed C10

How this works: Battery sizing is set by three separate limits, and the largest one wins: total stored energy in kilowatt-hours, continuous power output in kilowatts, and the instantaneous surge when a motor starts. This tool checks all three and tells you which one is driving your result, so you can see whether trimming a circuit or adding a battery is the better move. Figures use typical appliance wattages and standard duty cycles — a licensed electrician verifies the real numbers on site.

1
Battery Model
Powerwall 3 is the current generation for new installations

Both models store the same 13.5 kWh usable. The difference is how much power they can push out at once, which is what decides whether large loads like central air conditioning can run at all. Powerwall 2 is rated 5 kW real power continuous — the 5.8 figure often quoted is apparent power in kVA, not kW.

2
Circuits To Back Up
Check everything you want live during an outage
3
Runtime Target
How long the backup needs to carry you
4
Solar
Solar recharges the battery during a multi-day outage

Assumes roughly 4.2 kWh produced per kW of panel per day for Los Angeles, derated for real-world conditions. With solar the battery only has to carry the overnight load instead of the full day.

Your Result

Sized against energy, power, and surge

?

Pick the circuits you want backed up, then press Calculate to see how many Powerwalls your home needs.

The Method

Three limits decide how many Powerwalls you need

Most sizing advice online only looks at kilowatt-hours, which is why so many homeowners end up with a battery that dies early or trips out the moment the air conditioner kicks on. A correctly sized system has to satisfy all three of the constraints below at the same time, and whichever one demands the most units is the one that sets your answer.

Limit One

Stored energy

Each Powerwall holds 13.5 kWh of usable energy. Add up what your backed-up circuits consume over the outage window and divide by 13.5. This is the limit that determines how long you last, not whether things turn on. It is also the one limit you can extend cheaply: Powerwall 3 accepts up to three Expansion units that add 13.5 kWh each without adding output. Dropping an electric dryer or an electric water heater off the list moves this number more than any other single change.

Limit Two

Continuous power

A Powerwall 3 delivers 11.5 kW of continuous AC output per unit. A Powerwall 2 delivers 5 kW of real power — the 5.8 figure quoted in most places is apparent power in kVA, which is not the same thing. If every circuit you backed up could run simultaneously and the total exceeds that rating, the system overloads. This is the limit that decides whether whole-home backup is realistic or whether you want a critical loads subpanel instead.

Limit Three

Motor starting surge

Compressors and pumps draw several times their running wattage for the first second of startup. Powerwall 3 handles this with 15.4 kW of maximum continuous discharge off-grid and a published load start capability of 185 LRA, which covers most residential central air conditioning on a single unit. Powerwall 2 peaks at 7 kW for 10 seconds. A soft start kit on the condenser is frequently cheaper than adding a second Powerwall.

Backup Strategy

Whole-home backup or a critical loads panel

Once you know your numbers, the next decision is how much of the house the battery is wired to serve. This is an electrical design choice made at the panel, and it has a bigger effect on cost than most homeowners expect. Neither option is universally better — the right one depends on the gap between your continuous demand and your battery output.

Option A

Whole-home backup

Every circuit in the house stays live during an outage. Nothing has to be moved, nothing is off limits, and there is no thinking about which outlet works. It requires enough Powerwall capacity to cover the largest realistic simultaneous demand, which for a home with central air conditioning and electric appliances usually means two or more units.

This is the right call when your continuous demand comfortably fits inside your battery output, or when the household is not going to reliably manage its own consumption during an outage.

Option B

Critical loads subpanel

A smaller subpanel is installed and only the circuits you choose get moved onto it. Refrigerator, lighting, internet, a few outlets, the furnace blower, and medical equipment are the usual list. Everything else stays dark until utility power returns.

This keeps sizing honest and is often the difference between one Powerwall and three. It is the standard approach when a home has a large electric water heater, an electric dryer, or a five-ton air conditioner that would otherwise dominate the calculation for loads you do not truly need during an outage.

Reference

Typical backup loads in a Los Angeles home

These are the figures the calculator uses. Running watts is what the appliance draws in steady operation. Starting watts is the momentary surge as a motor spins up, which is what the battery has to survive. Hours per day is a realistic duty cycle during an outage, not continuous operation — a refrigerator compressor, for example, only runs about a third of the time.

LoadRunning wattsStarting wattsHours per day

Your actual equipment will differ. Older compressors and resistance-heat appliances draw more; newer inverter-driven equipment draws less. Jorge confirms real nameplate ratings during the site visit before anything is ordered.

Manufacturer Specifications

Powerwall 3 and Powerwall 2, as published by Tesla

These are the figures this calculator sizes against, taken from the Tesla product datasheets rather than from retailer summaries. One number is worth calling out because it is misquoted almost everywhere: Powerwall 2 is rated 5 kW of real continuous power. The 5.8 figure that circulates widely is apparent power in kVA, a different quantity, and using it will oversize what a Powerwall 2 can actually carry.

SpecificationPowerwall 3Powerwall 2
Usable energy13.5 kWh13.5 kWh
Continuous output (real power)11.5 kW AC5 kW
Apparent power, continuous11,500 VA5.8 kVA
Peak output15.4 kW off-grid maximum continuous discharge7 kW for 10 s, off-grid
Motor start capability185 LRANot published
Efficiency89% solar to battery to home, 97.5% solar to home90% round trip
Power scalabilityUp to 4 Powerwall 3 unitsMultiple units supported
Energy scalabilityUp to 3 Expansion units, 7 units totalNot applicable
Solar inputUp to 20 kW DC across 6 MPPTsAC coupled

Note on the 20 kW figure: that is Powerwall 3's solar DC input rating, not its output. It is frequently repeated as though it were a surge output number, which it is not. The output figures that matter for backup sizing are 11.5 kW continuous and 15.4 kW off-grid maximum continuous discharge. The 15.4 kW rating is only available when the on-grid rating is configured at 11.5 kW, and requires an 80 A breaker.

Los Angeles Context

Why homeowners here are adding battery backup

Los Angeles outages tend to be short but clustered. Summer heat waves put sustained strain on the grid across the San Fernando Valley, and rotating outages during peak demand are a normal part of a hot week in Van Nuys, Woodland Hills, and Chatsworth. In the foothill communities — Sunland, Tujunga, La Canada Flintridge, Porter Ranch — public safety power shutoffs during high wind and fire conditions can run considerably longer, which is where multi-day runtime planning actually matters.

The practical consequence is that most Los Angeles homes are sizing for two different scenarios at once: a short outage where the goal is that nothing in the house is disrupted, and a long outage where the goal is that the refrigerator, lighting, internet, and medical equipment keep going for days. Those two goals pull in different directions, and the honest answer for many homes is a critical loads panel plus solar rather than a large bank of batteries covering everything.

Installation is permitted work. Battery storage requires a permit and a utility interconnection application, and the jurisdiction depends on the address — LADBS handles everything inside City of Los Angeles limits, while independent cities like Burbank, Glendale, and Pasadena run their own building departments and their own municipal utilities. LED Guys handles the permit filing and the interconnection paperwork as part of the installation.

If your panel is older or already near capacity, battery installation often surfaces a panel problem first. Our Home Electrical Load Calculator is the faster way to check that side of things, and panel upgrades are handled in-house. If you are also planning EV charging, see Tesla charger and Powerwall installation — combining the two jobs into one permit and one visit is almost always cheaper than doing them separately.

FAQ

Powerwall sizing questions

How many Powerwalls do I need for my house?
It depends on three separate limits, not one. First, stored energy: each Powerwall holds 13.5 kWh usable, so your daily backup energy divided by 13.5 sets a minimum. Second, continuous power: a Powerwall 3 delivers 11.5 kW continuous, so if everything you back up can run at once and exceeds that, you need another unit. Third, motor starting surge: air conditioners, well pumps, and pool pumps draw far more current for the first second than they do running. Most Los Angeles homes backing up essentials only land at one Powerwall. Homes backing up central air conditioning typically need two or more.
What is a critical loads panel and do I need one?
A critical loads panel, sometimes called a backup subpanel, is a smaller panel that holds only the circuits you want running during an outage. The rest of the house stays dark. It is the right choice when your whole-house demand is well above what your battery can deliver continuously, because it keeps the sizing honest and the cost down. Whole-home backup keeps every circuit live but requires enough Powerwall capacity to cover the largest realistic simultaneous demand. LED Guys determines which approach fits during the on-site visit.
Can a Powerwall run my central air conditioner?
Often yes, but it is the single load that most changes the answer. A three-ton central air conditioner draws roughly 3,500 watts running but can pull 9,000 watts or more for the first second as the compressor starts. Powerwall 3 is rated for load starting up to 185 LRA and 15.4 kW of maximum continuous discharge off-grid, which covers most residential central air conditioning on a single unit; a five-ton system or an older compressor without a soft starter frequently pushes the requirement to two units. Powerwall 2 is far tighter here, peaking at 7 kW for 10 seconds. Adding a soft start kit to the condenser is often cheaper than adding a second battery, and it is one of the first things we check on site.
How long will a Powerwall last during an outage?
Runtime is stored energy divided by average draw, not peak draw. A single Powerwall holding 13.5 kWh running a 500 watt average load lasts about 27 hours. The same battery running a 2,000 watt average load lasts under 7 hours. This is why the circuit list matters more than the battery count: dropping an electric dryer and an electric water heater off the backup list can double your runtime without spending anything.
Does solar change how many Powerwalls I need?
Yes, significantly, for outages lasting longer than one day. With solar tied into the battery, the array recharges the Powerwall during daylight, so the battery only has to carry the overnight load rather than the full 24-hour load. For a single-day outage solar helps modestly. For a multi-day outage it is often the difference between one Powerwall and three. Without solar, every kilowatt-hour you use has to already be in the battery when the power goes out.
Is this calculator a substitute for a professional assessment?
No. This tool uses typical appliance wattages and standard duty cycle assumptions to get you a realistic starting number and to show you which constraint is actually driving your result. Your real numbers depend on your specific equipment, panel configuration, service size, and how your circuits are grouped. A licensed electrician has to verify the panel, confirm the backup strategy, and handle the permit and utility interconnection before installation.

Certified by Tesla Energy for Wall Connector and Powerwall

The Tesla Energy Certified Installer designation identifies professionals who have completed Tesla Energy training, hold deep technical knowledge of Tesla Energy products, and deliver the highest-quality installations for Tesla customers. LED Guys holds this certification for both product lines — a combination very few Los Angeles electricians can offer.

Tesla Wall Connector installed by LED Guys in Los Angeles

Wall Connector Certified Installer

LED Guys is certified by Tesla Energy to install the Tesla Wall Connector and Universal Wall Connector to Tesla's own installation standards. That certification covers the full scope of a residential or commercial Wall Connector project: conductor sizing per NEC Article 625, dedicated circuit breaker selection, conduit and wire routing, GFCI breaker protection per NEC 625.54, and proper grounding and bonding at the panel.

After the physical installation, Jorge commissions the Wall Connector through the Tesla One app — connecting the unit to Wi-Fi, registering it to your Tesla account, setting the output amperage to match your circuit, and configuring access controls if you want to restrict charging to authorized vehicles. For multi-vehicle homes and commercial sites, we configure Power Sharing so up to six Wall Connectors intelligently share a single circuit without exceeding its capacity.

The Wall Connector delivers up to 48 amps on a 60-amp circuit, the fastest Level 2 charging Tesla offers for home use. Its built-in amperage adjustment (12, 16, 24, 32, 40, or 48 amps) means it works with any panel — from a 100-amp service with limited headroom to a 400-amp commercial main. Tesla's NACS connector is now the North American charging standard (SAE J3400), so the Wall Connector charges Teslas natively and other EV brands via adapter or native NACS port. The Universal Wall Connector adds a built-in J1772 connector for households with mixed EV brands.

Powerwall Certified Installer

Certified for Tesla Powerwall home battery installations — whole-home backup, Backup Gateway integration, solar coordination, and Charge on Solar setup so your Tesla charges from excess solar production automatically.

Powerwall installations include gateway wiring, backup circuit selection based on your household's critical loads, and utility interconnection paperwork for LADWP net metering. Jorge configures Storm Watch, self-consumption mode, and time-based control through the Tesla app so your system responds automatically to grid conditions and rate schedules.


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GlowbackLED Trusted Lighting Installer

LED Guys is a GlowbackLED Trusted Lighting Installer — a designation earned by electrical contractors who consistently deliver high-quality installations of GlowbackLED's custom LED lighting systems. GlowbackLED is a US-based designer and manufacturer of architectural LED solutions: custom LED panels, recessed and pendant linear fixtures, LED aluminum channels, and cabinet lighting built to specification for high-end residential and commercial projects.

When a project calls for custom-fabricated LED lighting — a coffered ceiling with built-in linear channels, back-lit niches, under-cabinet strip lighting with dimmer controls, or a commercial lobby with signature pendant fixtures — Jorge sources components from GlowbackLED and installs them to their technical specifications. Every GlowbackLED installation includes proper driver placement, dimmer compatibility verification, and clean concealed wiring so the finished result matches the design intent.

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